Devices for collecting diffuse electromagnetic radiation by utilizing the fluorescence are known. The core of the collectors is a medium which has a greater optical density than its surroundings and which contains centers which are capable of fluorescence (German Offenlegungsschrift No. 2,620,115).
The economic applicability of a light-collecting system is to a large extent determined by the usefulness of the dyestuff employed therein as a light converter. Extreme demands are made on the optical quality of this dyestuff [Appl. Phys. 14 123-139 (1977)].